As an R.N. I have worked at several locations and the orders for enemas until clear are usually different in each place but the idea and process are very similar.
If the doctor does not specify the solution hospital policy might. SSE's have been used, (usually for the first enema only.) Saline is probably the most commonly used solution. The first treatment is usually the most uncomfortable for the patient but it depends on the volume ordered as well as the solution. In the 70's giving 1500-2000ml of a fairly concentrate castile soap was used and given by having the patient begin in the knee chest position and then roll to the right side to fill the ascending bowel. This can be quite uncomfortable but usually produces a very large movement. Even though this sounds uncomfortable it can be the easiest for the patient in the long run because it can reduce the number and volume of subsequent enemas. Back then some facilities specified saline instead of soap and this worked but sometimes not as well.
After the first enema most facilities used saline for follow up enemas. Again if the quantity was not specified in the order nursing judgment was used. 1500ml of warm saline was a common amount for the average adult. We usually gave this in the knee chest position. This enema usually produced some formed stool but far less than the first treatment.
Usually a third enema was required but for some patients the second enema produced only flecks of stool and brown water and again nursing judgment and hospital policy had to be used. A third enema was usually 1000-1500ml of saline given in the left lateral position. By this time the patient was becoming exhausted so it was lucky for them that the result was nothing but brown water.
We would stop at this point per hospital policy unless we had clear, confirmed orders from a physician to give more if the result was anything but brown water. Most of the time the prep was done the evening before the BE and frequently the physician ordered a Dulcolax suppository in the morning. Some radiologists ordered a Fleets Enema in the morning instead of the suppository. Again, a lot depended on the physicians orders and hospital policy. Frequently hospital policy was to stop after three enemas and get a new order from the physician.
I know that this is a long response, but I tried to cover the topic clearly. I invite other nurses or physicians to comment or enlarge on this if they feel necessary.
I would be happy to give you this procedure if I were in your area. I believe that your would find it interesting and probably since you are on this site it would be fun